In the recent image forming apparatus, a transfer unit is provided, which transfers an image formed on an image carrier such as an intermediate transfer belt onto a recording medium by causing the recording medium to be in contact with the image carrier. The recording medium having the image formed on a first side thereof is transferred to a fixing unit. In order to perform duplex printing, the recording medium that passes the fixing unit is transferred to a re-transfer unit. The re-transfer unit reverses the side of the recording medium, and re-transfers the recording medium to the transfer unit.
The fixing unit, which is provided to heat the recording medium, is usually provided with an oil applying unit that applies oil to a fixing member in order to prevent the toner from being adhered to the surface of the fixing medium. When performing the duplex printing, the recording medium that is sent to the fixing unit for fixing the image onto the first side of the recording medium may absorb the oil on the surface of the fixing member as it passes the fixing member. When the recording medium is re-transferred by the re-transfer unit, the oil absorbed by the recording medium may be transferred to the image carrier or a contact member that is provided near the image carrier such as a transfer roller. The oil transferred to the contact member may be further transferred to the image carrier as the recording medium is transferred from a fixing nip. When more oil is adhered to the image carrier, the amount of toner that is formed on the surface of the image carrier greatly decreases. As a result, an undesired blank section may be observed in the image of the printed recording medium.
The Japanese Patent Application Publication No. 2004-93999 describes an image forming apparatus capable of suppressing the occurrence of a blank section in a printed recording sheet. A plurality of toner images including the yellow toner image, the magenta toner image, the cyan toner image, and the black toner image are sequentially formed on the surface of a photoconductor. The toner images are sequentially transferred to the surface of an intermediate transfer belt functioning as an image carrier to generate a composite toner image. The composite toner image is transferred to a first side of a recording sheet by a transfer device. The recording sheet having the image formed on its first side is transferred to a fixing device to fix the image. The recording sheet having the fixed image formed on its first side is re-transferred to the transfer device by a re-transfer device. The transfer device transfers a composite toner image on a second side of the recording sheet. At this time, the oil adhered to the recording sheet at the time of fixing may be transferred to the intermediate transfer belt from the recording sheet. The oil is removed from the surface of the intermediate transfer belt when the printing job is completed. When the printing job is completed, the image forming apparatus performs oil removal processing to remove the oil from the surface of the intermediate transfer belt. A toner applying device, which is separated from the intermediate transfer belt at the time of performing the printing job, is made in contact with the intermediate transfer belt. The toner applying device includes an applying brush that is rotatably driven, which is made in contact with toner stored in a toner container and the intermediate transfer belt. With this applying brush, the toner is applied to the surface of the intermediate transfer belt. The toner that has been applied to the surface of the intermediate transfer belt absorbs the oil attached to the surface of the intermediate transfer belt. A cleaning device, which is provided to remove residual toner resided on the surface of the intermediate transfer belt after passing the transfer device, is made in contact with the intermediate transfer belt. The toner that has absorbed the oil from the surface of the intermediate transfer belt is scraped off from the surface of the intermediate transfer belt by the cleaning device. As the oil is removed from the surface of the intermediate transfer belt, the occurrence of the blank section in the printed recording sheet may be suppressed.
However, when performing the duplex printing especially for a large number of recording sheets, the above-described image forming apparatus may not be able to suppress the occurrence of the blank section. When the duplex printing job is being performed, the above-described oil removal processing cannot be performed as the applying brush cannot be made in contact with the intermediate transfer belt on which the toner image is formed.